Interplay between energy and geometry of parallel-displaced interactions in S8 dimer structures

Elemental sulfur exists in various allotropes, with the most stable form being the cyclic S8 allotrope. Although the orthorhombic alpha-S8 modification has been extensively studied, there has been no systematic analysis of non-covalent interactions between sulfur rings and their nature, geometries, and energies until now. To achieve this, we conducted a comprehensive study analyzing all crystal structures from the Cambridge Structural Database involving contacts between S8 molecules. Then, we combined those data with high-level quantum chemical calculations. The results revealed that the preferred orientation in the crystal structures is parallel-displaced. Quantum chemical calculations supported this finding, demonstrating that the most stable interaction in the S8 dimer occurs when two molecules adopt a parallel-displaced geometry resembling the one observed in the crystal structures. The interaction in the S8 dimer is very strong, with an energy of-8.70 kcal/mol, calculated at a highly accurate CCSD(T)/CBS level. In this stacking-like geometry, multiple S...S interactions can form, and the NCI analysis indicated an overlap of large surfaces of interacting molecules, significantly contributing to the system's stability. The SAPT2+(3) energy decomposition analysis showed that the predominant attractive force between two S8 molecules is dispersion, although the electrostatic component is also significant. This research holds importance in understanding the molecular assembly in solid sulfur and in systems that exhibit interactions between S8 molecules.